I like the idea. Region types are already there and it'd be more fun if they influence in some way the outcome of the battles.
Anyway I think it'd be better if this mix to other BM traits: for instance I'd give a bonus to Archers who are defending a woodland region of their realm and they're set to Sentries because with this setting it's assumed that they know well the terrain.
Generally I 'd not give automatic bonuses, players have to do something in order to get this gain.
